Think about it. Would it even be worth it? Considering that poly's study load is already quite heavy if one intends to get a GPA of 3.6 and above to be eligible for most university courses. Meaning that you could only do it post-poly. 2 years of NS + 3 years more of doing nothing but one O-level subject. Assuming one does not work as well. Then yes, a person can waste 3 years of his life to attain a perfect O level score. Then can get into NUS. Wooohooo. That is IF that person can do it. Just because a person takes ONE subject a year doesnt mean he can still achieve an A1. The amount of effort, money and time wasted just to get a perfect score spanning over 5 years is not worthwhile at all. The returns is just an extra say 5-10% for his admissions. If a person has serious ego issues that he die die MUST get into NTU/NUS then he might consider it. Otherwise, the rest of the rational bunch of applicants would not go to such extreme lengths.
I frankly speaking wouldn't say a poly workload as being heavy but than, to each his own. It depends on what you study as well, because there are overlaps between o level subjects and poly subjects
For instance, you could take up o level POA if you were an accounting student, or physics if you went into engineering. There are even courses in polytechnic that teach additional maths.There will be overlaps, not completely but to a certain extend.
Now back to my point about there being a difference. Imagine taking poa after you take a few years of accounting modules, or taking maths or physics after studying it at a poly level for a few semesters..Do you think they wouldnt know you would have studied this in poly?